The medical industry has developed a wide range of uses for implantable electrical stimulators, some are well developed while some are still experimental. Uses include pacemakers, cochlear stimulators for the deaf, retinal stimulators for the blind, and muscle stimulators to cure paralysis, chronic pain, sleep apnea, reduction of spasticity in limbs and eyelid droop.
In each of these technologies, the amount of current provided to a given electrode must be externally controllable. External control is typically provided by a digital signal transmitted to the implanted device. A small efficient and flexible digital-to-analog converter (DAC) and driver are required to convert the transmitted digital signal to a current level at each stimulator electrode.
These devices very quickly become large and complex. For example, a retinal stimulator for the blind requires one electrode for each pixel of light perceived. The current on each electrode, must change dynamically, with changes in the intensity of the light signal provided to the retina. A large number of electrodes, and a large number of corresponding DACs and drivers, are required to provide even limited vision. In addition to the varying signal relative to perceived light, the perception threshold and maximum tolerable level vary from person to person. Because the perceptible range is different for each individual, a DAC with a broad enough range to work for all people, would also provide levels unusable for many or those people. As the surgery for implanting such devices is quite complex, it would be an unreasonable burden on a patient to switch out a DAC for one with a different range.
One DAC is described in U.S. Pat. No. 6,181,969, (“Gord”). Gord discloses an efficient array of DACs for a cochlear stimulator. However, the Gord device requires preset devices which, when selected, provide a predetermined current level. A finite set of current levels are available. Once a device is implanted, it is impossible to change the range of selectable outputs. By summing the output of each device, it is possible to compound the possible output currents by two (i.e. 2 devices—4 levels, 3 devices—8 levels, 4 devices—16 levels, etc.). To achieve a reasonable level of sight for most people, would require an implant so large and complex, that it could not be implanted in or near the eye.
What is needed is a simple, efficient, and flexible DAC and driver that can provided a wide range of current levels. It is also important that a large number of DACs and drivers be manufacturable in a small package at a reasonable cost.